The present invention relates to an uninterrupted power supply (UPS) system and more particularly to an UPS systems having an improved power factor correction circuit.
It may be explained here that computer work stations require ever increasing power requirements, especially those with advanced graphic displays. These power requirements are approaching 1000 to 1200 watts of power. In theory this should not present a problem because 1725 watts of power should be available from a 15 amp, 115 volt wall outlet. In the United States, Underwriters Laboratories, however, limits current draw to 12 amps and, therefore the power to 1380 watts. With the efficiency of a typical power supply being 85%, a power factor of 0.7 and a low line voltage of 110 volts, the maximum power which can be drawn from the standard wall outlet becomes less than 800 watts. A power factor of 0.7 effectively reduces the power available from the AC utility power source by about 30%. It is for this reason that there is an increasing need to provide power supplies with improved power factor correction. Under so-called "brown out" conditions, i.e. the utility reducing the voltage available by 5%, the power that can be drawn is even further reduced to less than 700 watts.
Another factor promoting the need for improved power factor correction is that in Europe, after 1992, electronic equipment drawing more than 300 watts of rectified AC line power and driving a capacitive input filter must have power-factor correction.
This emphasis on power factor correction is also applicable to UPS systems. There is disclosed in copending application Ser. No. 366,098, assigned to the same assignee as the subject application and which is hereby incorporated by reference herein, a UPS system which maintains the integrity of the neutral conductor of the AC utility power source while eliminating the 50 or 60 Hz transformer thereby actually reducing the overall cost, size and weight of the UPS system. The power factor correction circuit in accordance with the present invention has particular application to the UPS system disclosed in copending application Ser. No. 366,098.